1. Field of the Invention
The present invention relates to a noise reducing device, a noise reducing method, a program for noise reduction processing, and a noise reducing audio outputting device.
2. Description of the Related Art
With the spread of portable type audio players, a noise reducing system that reduces noise of an external environment and thus provides a listener with a good reproduced sound field space in which the external noise is reduced has begun to be spread for headphones and earphones for the portable type audio players.
An example of this kind of noise reducing system is an active type noise reducing system that performs active noise reduction and which basically has the following constitution. External noise is collected by a microphone as acoustic-to-electric converting means. A noise reducing audio signal of acoustically opposite phase from the noise is generated from an audio signal of the collected noise. The generated noise reducing audio signal is acoustically reproduced by a speaker as electric-to-acoustic converting means, whereby the noise reducing audio signal and the noise are acoustically synthesized. Thus the noise is reduced (see Japanese Patent No. 2778173, hereinafter referred to as Patent Document 1).
In this active type noise reducing system, conventionally, a part for generating the noise reducing audio signal is formed by an analog circuit (analog filter), and is fixed as a filter circuit that can perform some degree of noise reduction in any noise environment.
In addition, a headphone device has been proposed which includes a noise reducing system employing an adaptive filter using adaptive processing and which can reproduce music even in an environment with a high level of external noise in a state of the noise being reduced (see Japanese Patent No. 2867461, hereinafter referred to as Patent Document 2).
The noise reducing system of a noise reducing headphone described in Patent Document 2 automatically sets the adaptive filter to an optimum filter using adaptive signal processing. A microphone for collecting external noise is provided on the outside of a headphone casing, and a microphone for collecting the sound of a residual (error) component as a result of acoustic synthesis based on the adaptive signal processing is provided inside the headphone casing.
In the noise reducing system using the adaptive processing, a residual signal from the microphone provided within the headphone casing is analyzed, and the adaptive filter is updated, whereby adaptive noise reduction is performed on the external noise.